The present invention relates generally to carriage shuttling systems and, more particularly, to shuttling systems for shuttling the print head of a dot matrix line printer.
While, as will be better understood from the following description, the present invention was developed to shuttle the print head of a dot matrix line printer and, thus, is expected to find its primary use in such printers, it is to be understood that the invention can be used to shuttle the carriages of other mechanisms requiring or desiring nearly constant velocity shuttle motion through most of the stroke.
Various types of dot matrix line printers have been proposed and are in use. In general, dot matrix line printers include a print head comprising a plurality of dot printing mechanisms, each including a dot-forming element. The dot forming elements are located along a line that lies orthogonal to the direction of paper movement through the printer. Since paper movement is normally vertical, the dot forming elements usually lie along a horizontal line. Located on the side of the paper remote from the dot forming elements is a platen and located between the dot forming elements and the paper is a ribbon. During printing, the dot forming elements are shuttled back and forth. As the dot forming elements are shuttled, they are actuated to create dots along the print line defined by the dot forming elements. The paper is incremented forwardly after each dot row is printed. A series of dot rows creates a row of characters or a graphical image.
The dot forming elements of contemporary dot matrix line printers are small anvils located on one end of an electromagnetically actuated hammer. The hammers are normally held in a retracted position by magnetic force. Release is created by the application of a pulse to an electromagnetic coil that produces a magnetic field that counteracts the retracting field. The dot forming element, hammer, retracting magnet, release coil, and related elements form a dot forming mechanism. The dot forming mechanisms may be grouped in sets of two or more and mounted on a carriage such that the dot forming elements are spaced apart and offset in a predetermined manner. See, for example, U.S. Pat. No. 4,351,235, titled Dot Printing Mechanism for Dot Matrix Line Printers.
Shuttling of the dot printing elements back and forth is accomplished by translating the carriage. In one type of line printer, the carriage is reciprocally mounted on a frame by a suitable support mechanism, such as a linear bearing. The carriage shuttle motion is a consequence of a crank driving either a connecting rod or a Scotch yoke. The axis of rotation of the crank is coaxial with the axis of rotation of a heavy flywheel. The heavy flywheel supplies and absorbs the energy required to accelerate and decelerate the carriage with only negligible changes to its rotational speed. While the near-constant angular velocities that result from heavy flywheel carriage shuttle systems have some advantages, they also have a number of disadvantages.
A carriage that shuttles back and forth must do so with a velocity that changes with time because the carriage must stop and reverse direction at the end of each shuttle stroke. In a heavy flywheel shuttle system, the energy stored in the rotational motion of the flywheel is substantially greater than the energy stored in the translational motion of the carriage where rotational kinetic energy is equal to xc2xd times the moment of inertia times the angular velocity, in radians, squared and where the translational kinetic energy is equal to xc2xd times the mass times the linear velocity squared. As a result, the change in the angular velocity of the flywheel due to the deceleration of the carriage at the end of the print stroke is negligible. The nearly constant angular velocity of the flywheel causes the shuttle mechanism to accelerate from zero velocity at its end of a shuttle stroke to peak velocity near the middle of the shuttle stroke. The end result is a carriage velocity profile that is sinusoidal for a Scotch yoke mechanism and nearly sinusoidal for a crank/connecting rod/slider mechanism.
A printer whose carriage has a sinusoidal velocity profile is significantly slower than a printer having a more constant velocity profile. This result occurs because the average velocity of a sinusoidal velocity profile carriage is 64% of the peak velocity. The peak shuttle velocity for a printer is limited by the maximum firing rate of the dot making mechanism, while the overall printing speed is a function of the average shuttle velocity. The more constant the carriage velocity and, thus, the flatter the velocity profile during the printing portion of a sweep, the greater the printer speed. Therefore, it is desirable that the carriage of a dot matrix line printer have a substantially constant velocity profile during the print portion of a shuttle stroke.
In the past, various types of dot matrix line print carriage shuttle systems for achieving a more constant shuttle velocity profile have been proposed. One such proposal is to use a cam/cam follower carriage shuttle system to convert a constant angular input velocity to a flattened near-constant output velocity. However, cam/cam follower carriage shuttle systems are undesirable in a dot matrix line printer because they are subject to a high degree of mechanical wear. More specifically, dot matrix line printers, particularly high speed dot matrix line printers, require precision positioning of the printer head at the time the dot-forming elements are actuated by their related actuating mechanisms. Mechanical wear reduces the precision with which the print head can be positioned. As print head positioning precision drops, dot misregistration increases. As a result, printed characters and images are distorted and/or blurred. Distorted and/or blurred images are, of course, unacceptable in environments where high quality printing is required or desired.
Another prior proposal directed to providing a dot matrix line printer carriage having a more constant shuttle velocity profile is to mount the carriage on a continuous band driven by a motor. A major problem associated with a band-driven system is that the motor must stop at the end of each stroke and reverse the direction of the carriage. Reversing the direction of a carriage requires a large energy input to the motor because the motor must control the deceleration of the carriage and then, immediately thereafter, the acceleration of the carriage. Another problem associated with belt-driven systems derives from the fact that the stroke of the shuttled carriage is not mechanically restrained and, therefore, the extremes of travel are not defined. Because the extremes of travel are not defined, band-driven systems require mechanical stops.
Linear motors have also been employed in the past to produce near-constant shuttle velocities. But linear motors adequate to drive the dot matrix line printer carriage are undesirably large and expensive.
Thus, there exists a need for a relatively simple and energy efficient dot matrix line printer carriage shuttle system that has a substantially constant velocity profile across the print portion of the shuttle stroke. The present invention is directed to fulfilling this need.
In accordance with the present invention, a low rotational inertia shuttle system that is ideally useful in shuttling the print head of a dot matrix line printer is provided. The low rotational inertia shuttle system includes a mechanism to convert angular input motion to linear output motion. Such a mechanism could consist of a Scotch yoke, a crank-connecting rod-slider or a cam and cam follower. The specific requirement is that for a constant angular input velocity, the output velocity is sinusoidal or near-sinusoidal. A duplicate mechanism may be present to drive a counterbalance. The inertia of the rotating components is purposely minimized with respect to the mass of the carriage and the counterbalance. The low rotational inertia shuttle system further includes a source of mechanical power in the form of a rotary motor that causes the input to the mechanism to rotate. The rotary motor is controlled in such a fashion that it only supplies energy to overcome the frictional losses of the shuttle mechanism. The energy to accelerate and decelerate the carriage subtracts from and adds to the rotational energy of the rotating elements of the system. Because the rotational energy of the rotating elements (reciprocating mechanism) is on the same order as the energy in the reciprocating carriage mass and counterbalance, removal and addition to the rotational energy of the rotating elements results in a significant slowing of the angular input speed during mid-stroke and a significant speeding up of the angular input speed at the ends of the stroke. As a result of the changes in input angular speed, the output linear speed is increased at the ends-of-stroke and decreased during mid-stroke when compared to the output speed of a printer having a large rotational mass, such as a flywheel. This flattens the sinusoidal output velocity and makes the speed more constant over the dot forming portion of the shuttle stroke. Thus, by purposely minimizing the rotational inertia of the system, the output velocity is made more constant.
In accordance with further aspects of this invention, the reciprocating mechanism of the low rotational inertia shuttle system is a linkage that includes dual throw cranks and horizontally opposed connecting arms pivotally disposed between the carriage and counterbalance. This linkage converts a constant angular input velocity to a near-sinusoid linear output velocity.
In accordance with yet other aspects of this invention, the low rotational inertia shuttle system includes a stepper motor connected to the dual throw cranks. The stepper motor is controlled by a programmed control system. The program causes the stepper motor to track the natural tendency of the input angular velocity to speed up and slow down so as to maximize the linearity of the carriage velocity profile in the range where printing occurs.
A low rotational inertia shuttle system formed in accordance with the present invention has several advantages over shuttle systems used in the past in dot matrix line printers. The purposeful minimization of the inertia of the rotational elements with respect to the mass of the carriage and counterbalance conserves the system""s kinetic energy and produces a carriage velocity profile that is substantially linear throughout the print range of the dot matrix line printer. While the low rotational inertia shuttle system can be driven by a DC motor, the use of a stepper motor to selectively add or remove energy maximizes the linearity of the carriage velocity profile. Thus, a printer incorporating the present invention requires less energy to run and, therefore, is cheaper to build and to operate than those currently available. Such a printer can be operated at higher speeds without a loss of print accuracy. As a result, printer throughput is enhanced.